This invention relates to a battery separator for use in flooded cell type lead acid batteries.
In a flooded cell type lead acid battery the positive and negative electrodes or "plates" are separated by a battery separator. The battery separator typically has "ribs" or protrusions extending from at least one planar face of the separator. Such ribs are formed in one of several ways: the ribs can be formed integrally with the backweb of the separator; the ribs can be subsequently applied to the backweb as a bead of the same or different material as the backweb; or the ribs can be formed by embossing the backweb. The ribs function to provide proper spacing between the plates and to provide a space wherein free electrolyte resides.
The battery separator currently used by most flooded cell type lead acid battery manufacturers is of the microporous polyethylene type. This type of separator has a composition consisting essentially of an ultra high molecular weight polyethylene, a filler (typically amorphous silica), a plasticizer (typically a processing oil), and certain minor ingredients such as an antioxidant, lubricant and carbon black.
Microporous polyethylene separator material is commercially manufactured by passing the ingredients through a heated extruder, passing the extrudate generated by the extruder through a die and into the nip formed by two heated calender rolls to form a continuous web, extracting a substantial amount of the processing oil from the web by use of a solvent, drying the extracted web, slitting the web into lanes of predetermined width, and winding the lanes into rolls.
Such separators and a method of manufacturing them are described in U.S. Pat. No. 3,351,495.
Microporous polyethylene separators typically have a configuration comprising a backweb having a predetermined thickness, and a plurality of parallel ribs spaced apart a predetermined distance and extending outwardly from one planar surface of the backweb. The ribs extend continuously in a longitudinal direction parallel to the edges of the separator material. The thickness of the backweb and height and spacing of the ribs is specified to the separator manufacturer by the battery manufacturer; the specifications are designed to maximize certain battery characteristics desired by the battery manufacturer. Starting-lighting-ignition ("SLI") lead acid batteries used, for example, in automobiles, tend to have separators that are thinner than "industrial" lead acid batteries used for standby power sources and traction devices.
It is also known to form "mini-ribs" between such "major" ribs to add stiffness to separator webs having thinner backwebs. Generally, such mini-ribs have a lower height than the major ribs and are spaced closer together. The height of such mini-ribs typically varies between about 0.006 inch and about 0.009 inch. The spacing of such mini-ribs varies between about 0.060 inch and about 0.250 inch.
Such ribs (both major and mini) are formed during manufacture of the microporous polyethylene separator by providing that one of the two heated calender rolls forming the nip through which the extrudate from the extruder is fed is engraved with grooves so that the ribs are formed as an integral part of the separator web.
There are many different specifications required by battery manufacturers relative to rib size and rib spacing. In manufacturing separator material to meet customer requirements, almost every change in rib size and spacing requires that the separator manufacturer shut down its manufacturing line in order to remove the engraved roll that had been in use to fill the prior order and to insert a differently configured engraved roll capable of producing the rib size and spacing required for the new order to be filled. Manufacturing time is lost during such shut-down and extra scrap material is generated during start-up of the line.
In addition, integrally formed ribs in the polyethylene type separator undergoes extraction along with the backweb and, because it has relatively more volume than a portion of the backweb occupying the same planar surface area, generally the ribs retain more processing oil than the backweb, thereby raising the overall electrical resistance of the separator.
In commonly owned copending U.S. patent application Ser. No. 08/646,764, filed May 8, 1996, now U.S. Pat. No. 5,679,479, there is disclosed a battery separator having a longitudinal dimension, a width dimension perpendicular to said longitudinal dimension, upper and lower planar faces, and a plurality of ribs (at least three) projecting from at least one planar face, said ribs extending in a direction substantially parallel to the longitudinal dimension (axis) of the separator, each of the ribs being formed of a plurality of individual projecting embossments forming a corrugated structure comprised of alternating ridges and furrows. The ribs may extend from one or both planar faces of the separator. Where the ribs extend from both planar faces, adjacent projecting embossments (ridges) on one planar face are separated by an indentation (furrow) which forms a projecting embossment (ridge) on the other planar face of the separator. Where ribs extend from both planar faces of the separator, the ribs projecting from one planar surface may have a height equal to or different from the height of the ribs extending from the other planar surface.
While the separator described in Ser. No. 08/646,764, now U.S. Pat. No. 5,679,479, performs very well where the rib height does not exceed about 0.030 inch, it has been found that with rib heights in excess of about 0.030 inch compression resistance of the ribs becomes less satisfactory. By "compression resistance" is meant resistance to a compressive force applied to the tops of the ribs. In commonly owned patent application Ser. No. 08/837,286, filed Apr. 11, 1997, now U.S. Pat. No. 5,789,103, there is disclosed an improvement to the separator described in Ser. No. 08/646,764, now U.S. Pat. No. 5,679,479. The improved battery separator of the Ser. No. 08/837,286 patent application, now U.S. Pat. No. 5,789,103, employs an embossable base web comprised of a backweb having a plurality of submini-ribs extending from at least one planar face thereof, the base web being embossed with a plurality of major ribs, each major rib being substantially parallel to the longitudinal axis of the backweb and extending into at least one, and preferably two, adjacent submini-ribs to form a separator having improved compression resistance. The base embossable base web having submini-ribs thereon is separately disclosed and claimed in U.S. patent application Ser. No. 08/837,287 filed Apr. 11, 1997, now U.S. Pat. No. 5,894,055.
The separator disclosed in Ser. No. 08/646,764, now U.S. Pat. No. 5,679,479, and the improved separator disclosed in Ser. No. 08/837,286, now U.S. Pat. No. 5,789,103, have embossed (major) ribs that are disposed substantially parallel to the longitudinal dimension (axis) of the separator, and were designed for use in batteries having flat plates. It has been found that when attempting to use such separators to envelope (wrap) tubular plates the separator tended to nest down around the plate. In addition, acid (electrolyte) stratification and gas release were not satisfactory.
it is an object of this invention to provide a battery separator for use in a flooded cell lead acid battery having tubular plates; the separator having improved nesting resistance, reduced or eliminated acid stratification, and improved gas release.